Homeothermy, homothermy or homoiothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate environment (from Greek ὅμοιος homoios "similar" and θέρμη thermē "heat"). Homeothermy is one of the three types of thermoregulation in warm-blooded animal species. Homeothermy's opposite is poikilothermy. A poikilotherm is an organism that does not maintain a fixed internal temperature but rather fluctuates based on their environment and physical behaviour.

Homeotherms are not necessarily endothermic. Some homeotherms may maintain constant body temperatures through behavioral mechanisms alone, i.e., behavioral thermoregulation. Many reptiles use this strategy. For example, desert lizards are remarkable in that they maintain near-constant activity temperatures that are often within a degree or two of their lethal critical temperatures.